function H_t_f = Gen_time_frequency_channel(N,M,gs,L_set)
H_t_f = zeros(N,M); 
Fm = dftmtx(M);
Fm = Fm./norm(Fm);
Gn = zeros(M,M);
for n = 1:N
    for m = 1:M
        for l = L_set+1
            if(m >= l)
                Gn(m,m-l+1) = gs(l,m+(n-1)*M);  %42
            end
        end
    end
    H_t_f(n,1:M) = diag(Fm*Gn*Fm').';          %时域矩阵->时频域矩阵
end
end